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Offline dray

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Re: z axis moving up
« Reply #30 on: June 07, 2012, 07:53:09 PM »
Tweakie.cnc
Yes I've been trying anything that I could change but none of it effects the error that happens. Automation suggested a test using a pointer on the shaft of the stepper and a mark on the case. If I run a loop of say 50 cycles of back and forth for 1 revolution on any 1 axis at a time they all show an error in the up direction if that particular stepper was used on the z axis. This is run on only one axis at a time. Any suggestions? I have another bob with drivers on order just so I can eliminate that component. The error of .003"-.004" is very repeatable. It just seems to let the 000 position move ever so slightly with each rev.

dray

Offline RICH

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Re: z axis moving up
« Reply #31 on: June 07, 2012, 11:02:21 PM »
dray,
- I wanted to eliminate the power supply as the culprit by only only using it's power to run a single axis. So no power is being used by
the other drives.

- To eliminate the mechanical influences we are looking at pulses. Since you can't electronicaly count the pulses we are doing it
indirectly via a marker. It is not absolute as if you were using a counter. How well you can interpret the number of pulses is dependant on
the visual indication. I already explained the intent and you can measure form a simple disc.

- As said only move in one direction. NOT NOT back and forth  or  up and down.  I am isolating the influence of different rotor positioning
and want the movement as simple as possible. Changing direction can require additional pulses or even skipping which you cannot account for.

Once you have a base to relate to, by that I mean, the measured results of just moving in one direction and a comparison to moving in the the other direction. Then you can change some things, only one at time,....... to see if something makes a difference.
- swap drives, pulse duration, active settings, etc.
All of this imust done in a very disciplined way.

RICH





Offline dray

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Re: z axis moving up
« Reply #32 on: June 09, 2012, 11:20:49 AM »
Rich
Finally got some testing done hopefully in the right manner. I want to thank you for your patience. I am attaching a text file which explains the test as it was easier to write up in notepad.

dray

Offline RICH

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Re: z axis moving up
« Reply #33 on: June 10, 2012, 11:56:28 AM »
Lets do some math:
- For a 20" diameter disc, the circumference is 62.832"

- You commanded  a move of 1.5" in Mach, and since your steps per inch is 25600, then idealy 1.5x25600=38400 pulses need to be sent to the stepper for that linear movement.
   The DRO in Mach should have iindicated 1.5000" change in position. The mechanical indication of that movement is relative to the pointer position as compared to the
   the mark on the disc. Note that we are "relating" electrical to mechanical movement and idealy you would measure the pulses  sent out the pp, the pulses  recieved by the
  drive, the pulses sent from the drive to the stepper, and finaly use a physical indication of the rotor positioin . I have done the this using  a custom  input pulse counter accurate
   to parts per million which provides direct comparison. You don't have that kind of equipment, nor I, but the disc measurements although not exact,  are very indicative
   of the TOTAL pulses sent.

- One revolution of the disc requires 200 pulses and since you are microstepping, then, 200 x 8= 1600 total steps to rotate the disc 1 revolution.

- If 38400 pulses are recieved by the stepper, then 38400 PULSES  / 1600 PULSES PER rotation of the stepper  rotor = 24 rotations of the disc.

- The disc actualy rotated 24 times plus some additional amount ( .060" on the disc) which is equal to .3437 degrees too much ( approx).

- One degree on the disc, 1600 pulses per 360 deg per roatation= 1600 / 360 = 4.444 pulses / degree.
  The rotor turned too much, thus 4.444 pulses per degree x .3437 degrees = 1.5274 pulses  to manny. Well kind of ....readon....

Now to relate some of the above info:
- Not all drives are equal and thus there is non linearity associated with them. Not all steppers are equal on thier ability to position accurately in the microstepping.
  If you read the posted links by Ian in reply #13, you would understand that when a stepper recieves a pulse to make a small move it may take more than one since
  since it must overcome the detent torque and internal friction. Then depending on the magnetic field and position of the rotor it can after a few pulses then overmove.
  A good example of  this can be done using your disc to see how manny jogs it takes to move just .0001". May take more than calculated or rotor may move more than deisred.
  In fact each position will vary over the complete revolution, won't be exactly linear and you can't tell if the drive or motor has smore influence on this.
  This is just when moving in one direction, now apply the same thinking to a change in direction, and then consider that linearity will be different when going the other direction.
  Do you now understand why i only wanted to do one direction moves and i also wanted you to position the rotor so that it would not need to reserse direction?

 So you get some data for only a single direction move, then you get get some data on changing direction, then data on the other direction. You will then have an
understanding of how things relate and then a  base do do changes to see if  some settings will improve things.
Now remember that the  motor needs to ramp up to speed and then also ramp down to stop. An the power supply can have an impact on that and also positioning.
 Do now understand why I wanted you to do this at a slow feed rate and acceleration and only one drive at time?

BTW, pulses are on and off comming out of the PP, whole pulses, so there is no such thing as sending a 1.5274 pulse.
   
- The smallest move you can do, if all was ideal, is 1/25600=0.000039" for a linear move. So say, two pulses too manny were sent for example,
   then your axis  would be out of position by 0.0008". Is that repeatable for a one direction move?


I have lots of patience and also had problems with my mill when i started doing 3d cutting.
Your the one doing the testing ..........., ;)

RICH
 

Offline dray

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Re: z axis moving up
« Reply #34 on: June 10, 2012, 02:25:16 PM »
Rich
Thanks and I will try to get some more data. But right now am on a 5 day camping trip so it will have to wait till I return.

dray

Offline dray

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Re: z axis moving up
« Reply #35 on: June 27, 2012, 06:07:20 PM »
Hi Rich
Finally got started improving the 20 inch circle segment and a more accurate .1 inch scale on the machine frame. will send a photo of setup. seems like it always over shoots in the up direction and when going back down falls short of the previous point. With a 1 inch travel seems to average .1 inch error. I put a scope on the LPT cable on pins 6 & 7 and there is only 3.5 volts available and are very jittery when running some gcode. I think I saw a way to use a 5 volt pull up on these pin just have to find it again.

dray

Offline dray

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Re: z axis moving up
« Reply #36 on: June 29, 2012, 11:36:10 AM »
Hi Rich
I am attaching 3 file. 2 of which are photos. thanks
dray

Offline dray

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Re: z axis moving up
« Reply #37 on: June 29, 2012, 12:04:45 PM »

Hi Rich
We finally got it solved. That 20" dia. circle really shows up the errors The following settings have stopped the errors. Only the Z axis had power to the stepper during the testing. I am sending pictures of the pointer and scale used to determine the errors.

kernel 25000 hz
pulses/inch=25601 for Z axis
vel.= 20"/min., accel.= 2, step and dir.= 1 Us
set computer to "standard pc"
turned on the Sherline 1/2 pulse

I ran a 1" test up and back to zero 20 times with out any visible error.

We than cut a 5" diameter buffalo nickle very nicely in MDF.

Thanks to everyone for your help and suggestions, and especially you Rich, Mach 3 is an exellent program and at a very reasonable price.

dray

Offline RICH

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Re: z axis moving up
« Reply #38 on: June 29, 2012, 05:39:42 PM »
Happy you got it working right.
Have fun,
RICH